Disclosed is a radio frequency (RF) power calorimeter having a load electrically coupled to a RF input, a variable low-frequency power source electrically coupled to the load and configured to apply low-frequency bias to the load. The RF power calorimeter includes a thermal medium thermally coupled to the load. Additionally, the RF power calorimeter includes an outlet temperature sensor thermally coupled to the thermal medium, the outlet temperature sensor being positioned to measure the temperature of the thermal medium due to heating by the load. The RF power calorimeter also has circuitry configured to use temperature measurements of the thermal medium in thermal contact with an RF load in combination with the low-frequency bias to measure average power of an RF source electrically coupled to the RF input. Also disclosed in a method of measuring RF power using the RF power calorimeter.